Red maple is the most widely planted street tree in the Eastern US. Red maples in southeastern cities are plagued by extraordinary abundance and damage from gloomy scale insects, Melanaspis tenebricosa. In this research we examined how the urban heat island effect and other habitat features like impervious surface affect gloomy scale abundance on red maples. We then investigated how urban heat and scales combine to affect tree condition, functions like photosynthesis and growth, and ecosystem services like carbon sequestration and atmospheric cooling.

Research Approach

Adam Dale and Elsa Youngsteadt worked for several years to understand how urban heat and impervious surface affect scale insect abundance and red maple condition, growth, and ecosystem services. So far they have addressed four primary questions:

For decades, ecologists have debated the importance of abiotic factors, such as temperature, and biotic factors, such as predation and parasitism, in regulating herbivore abundance and distribution.

Warmer temperatures can decrease arthropod development time and increase survival and fitness. However, low vegetation density and complexity can also reduce natural enemy abundance and biological control. Our goal is to separate the effects of habitat complexity on temperature from its well-documented effects on natural enemies, to understand why M. tenebricosa thrive on urban trees.

We found that trees in the hottest urban habitats had three orders of magnitude more M. tenebricosa than the trees in the coolest urban habitats; some had up to 2241 individuals per 0.6 m (2 feet) of twig. We conclude that in our study, urban habitat characteristics such as high impervious surface cover increase herbivore abundance by making the habitat warmer, rather than less suitable for natural enemies as proposed in other studies.

Gloomy scale embryos. Photo: Adam Dale

Greater gloomy scale abundance could be due to greater fecundity at hot sites. Female M. tenebricosa were significantly larger and produced three times as many eggs in warmer than cooler urban habitats.

Vegetation structural complexity and ground cover did not increase natural enemy abundance or efficacy as predicted by other studies. Parasitoid abundance was greater in simpler habitats where scale insect abundance was also high. However, percent parasitism was also not affected by vegetation structural complexity or land use type so parasitoid function does not seem to be reduced at hot sites.

Our hypothesis was that urban heat increases M. tenebricosa abundance and that these two factors concurrently increase tree water stress and reduce tree condition.

To test this hypothesis, we measured tree midday water potential, trunk growth, and branch growth at various levels of scale insect abundance and temperature. Lastly, we used a standardized method for rating tree condition and a citywide tree inventory to compare trees at various levels of scale insect abundance and temperature within the city.

Our results show that urban warming increases water stress and scale insect abundance on street trees, and that warming and scale insect abundance each contribute to reducing tree condition. Less than 2°C increase in temperature reduces tree condition so that the hottest sites have over twice as many trees in poor condition than cooler sites, which represent conditions that may become more common as urban and global warming progress. Moreover, nearly 90% of red maple street trees are in less than excellent condition, which reflects the unsuitability of urban habitats for red maple trees.

Impervious surface increases tree drought stress by increasing atmospheric temperature and by reducing water availability to plant roots. The plant stress hypothesis proposes that drought can increase herbivore fitness and abundance by increasing the nutritional quality of plants and reducing plant defenses.

Adam using the Li-COR to measure photosynthesis, conductance, and transpiration.

Sap-feeding pests in particular often show a positive response to drought stress. Our objective was to determine how drought stress and warming interact to affect the fecundity and population growth of gloomy scale, a sap feeding scale insect pest. To do this, we measured gloomy scale body size, fecundity, and abundance over three generations on street trees that were watered twice per week and on unwatered trees.

We show that drought exacerbates the effect of warming such that gloomy scale produced over 17% more offspring on the warmest unwatered trees than the warmest watered trees, and over 65% more than the coolest watered trees. Gloomy scale abundance increased 200-fold across just over a 2°C increase in tree canopy temperature. This occurred over many years of infestation and exposure to the urban environment. Just one year of watering reduced drought stress and scale fecundity compared to unwatered trees.

Our results suggest that after 4 years of watering, lower fecundity will reduce gloomy scale abundance below that of trees that are not watered. Watering should also reduce gloomy scale colonization and population growth on newly planted trees. This emphasizes the importance of reducing water stress so that gloomy scale cannot grow to damaging populations.

The rate and severity of global climate change and urbanization are increasing, yet we still have inconsistent predictions about how heat and drought will affect herbivorous insects. Understanding how multiple climatic factors like temperature and drought act together to affect insect pests and their host trees is essential for managing forests and ecosystem services under climate change.

The first step in urban landscape integrated pest management (IPM) is planting the appropriate tree species for the conditions at specific urban sites. Unsuitable conditions can increase tree stress and pest abundance, limiting the ecosystem services trees provide.

Impervious surfaces cover around a planting site can stress plants by limiting root growth, raising temperatures, and creating drought-like conditions. We have shown with previous research that these conditions increase scale insect abundance and reduce tree condition and growth.

Probability curves illustrating the change in probability of finding a given tree condition rating as percent impervious surface around a planting site increases from 0% to 100%. Threshold lines are the average point at which each radius from the tree predicts a change in tree condition for all 13 measured radii. Each symbol indicates a different measured radius from the tree. Different colors represent different tree condition ratings.

We have shown that greater impervious surface cover leads to warmer temperatures, greater M. tenebricosa abundance, and worse A. rubrum street tree condition. Our analyses of impervious surface cover and tree condition indicate that red maple condition is most likely to be excellent or good if impervious surface cover is less that 32% within a 25m radius. At 33% to 66% impervious surface cover, trees were most likely to be in fair condition. Thus, red maples should be planted in these sites only if other factors like soil moisture or shade will mitigate the negative effects of impervious surfaces. Above 66% impervious surface cover, trees were mostly in poor condition. Red maples should not be planted in these sites.

Landscape architects and other planners can use ARCGIS or AutoCAD to measure impervious surface cover when designing a landscape with A. rubrum. For tree care professionals without software to analyze impervious surface cover we developed the ‘Pace to Plant’ technique.

Our ‘Pace to Plant’ technique provides a way to determine if a site is suitable for red maples. To measure impervious surface with ‘Pace to Plant’, stand at the tree site then take 25 steps at a 45 degree angle to the closest impervious edge. Count the number of steps that land on impervious surface. Repeat this 3 more times rotating 90 degrees each time for a total of 100 steps. Our analyses show that the number of steps out of 100 accurately estimates the actual impervious surface cover. Using impervious surface thresholds and the ‘Pace to Plant’ technique, landscape architects and other landscape professionals have the tools to plant the most common landscape tree in the eastern U.S. in more suitable locations.

Tools for Improving the Urban Forest

An IPM decision-making tool called ‘Pace to Plant’ has been developed from this research. It will help planners and urban forest managers get the right tree in the right place, reducing future maintenance costs and insecticide applications while increasing tree survival and services.

We are not the only scientists using cities as surrogates for climate change. However, this line of research is in its infancy. We conducted a literature review, led by postdoc Nora Lahr, to compile all the research we could find in which cities were used to predict the effects of climate change.

As a Masters’ student in the Frank lab, I study relatively small organisms called ground beetles. Ground beetles are used to monitor forest health because they are common, vary in food and habitat requirements, and are sensitive to human-caused disturbances.

Cities are hot and often dry. This makes the plants dry and unhealthy. But what about the animals? They can gain water by ‘drinking’ from moist soil or dew, or by eating plants that are mostly water. But what if they can’t find enough to drink?